This invention relates to a secondary battery protection circuit for use in a battery unit comprising at least one chargeable electric cell (secondary battery) such as a lithium ion cell and, in particular, to measures in a case where it is impossible to operate the secondary battery protection such as on short-circuiting of outputs thereof.
Among various types of chargeable electric cells, a lithium ion cell is particularly weak against overdischarge or overcharge. In this connection, it is essential to provide a secondary battery protection circuit for detecting an overdischarge condition and an overcharge condition to protect a secondary battery from the overdischarge condition and the overcharge condition. For this purpose, the secondary battery protection circuit comprises an overdischarge prevention mechanism and an overcharge prevention mechanism. In addition, the secondary battery protection circuit may detect an overcurrent condition during discharge of the secondary battery to protect the secondary cell from the overcurrent condition. In this event, the secondary battery protection circuit may comprise an overcurrent prevention mechanism as well as the overdischarge prevention mechanism and the overcharge prevention mechanism. However, description will be hereinuder directed to the secondary battery protection circuit comprising the overdischarge prevention mechanism and the overcharge prevention mechanism.
Such a secondary battery protection circuit is disclosed, for example, in Japanese Granted Patent Publication of No. 2,872,365 or JP-B 2872365 (which will later be called xe2x80x9cprior art referencexe2x80x9d) under the title of xe2x80x9cCHARGEABLE POWER UNIT.xe2x80x9d According to JP-B 2872365, the secondary battery protection circuit comprises a secondary battery, a discharge control switch, and a charge control switch. The discharge control switch and the charge control switch comprise first and second MOSFETs containing first and second parasitic diodes, respectively. The first parasitic diode is connected to the first MOSFET so that a forward direction of the first parasitic diode coincides with a charging direction of the secondary battery. The second parasitic diode is connected to the second MOSFET so that a forward direction of the second parasitic diode coincides with a discharging direction of the secondary battery.
However, in the manner which will later be described in conjunction with FIGS. 1 through 3 in detail, it is feared that the following problem occur in the above-mentioned prior art reference. That is, it is impossible to operate the secondary battery protection circuit on short-circuiting of outputs thereof.
It is therefore an object of the present invention to provide a secondary battery protection circuit which is capable of preventing the secondary battery protection circuit from being impossible to operate on short-circuiting of outputs thereof.
Other objects of this invention will become clear as the description proceeds.
On describing the gist of an aspect of this invention, it is possible to be understood that a method is for protecting a secondary battery by controlling, by turning a discharge control switch on and off, a discharging current flowing from the secondary battery through a load connected between a pair of external connection terminals. The discharge control switch has a control terminal.
According to the aspect of this invention, the above-mentioned method comprises the step of clamping the control terminal of the discharge control switch into a ground potential when the external connection terminals are short-circuited.
On describing the gist of another aspect of this invention, it is possible to be understood that a secondary battery protection circuit is for protecting a secondary battery by controlling, by turning a discharge control switch on and off, a discharging current flowing from the secondary battery through a load connected between a pair of external connection terminals. The discharge control switch has a control terminal.
According to the other aspect of this invention, the above-understood secondary battery protection circuit comprises a clamping circuit for clamping the control terminal of the discharge control switch into a ground potential when the external connection terminals are short-circuited.